If a sample of air is progressively cooled, its relative humidity would steadily increase i.e. the air would become relatively more moist. At some temperature, the air would become wet i.e. its relative humidity would become 100%. The air is then said to be saturated and the temperature at which this occurs is called dew point temperature of that sample of air. So, the dew point can be defined as that temperature of a sample of air at which it is 100% saturated so that if cooled below, it will give off its vapour. Dew point of a sample of air would depend on its temperature & relative humidity.